There
are several methods that can be used to map plant locations at a botanical garden
or arboretum. Regardless of the method, the goal is the same - determine the
plant's coordinates. This can be done using a tape to measure the distance from
the plant to known landmarks. Or, a total station electronic-optical surveying
instrument can be used. It accurately measures both distances and angles from
known landmarks and can be linked to a computer or data collector for automated
surveying, as in the case of the BG-Map Total Station
Interface.Another
technology for use in plant mapping is the Global Positioning System or GPS.

GPS technology continues to improve and decrease in cost.
GPS offers the advantage of efficient single person mapping - just roam around
the garden with the GPS receiver. The BG-Map GPS
Interface allows you to import GPS data directly into BG-Map for
automatic mapping of plants and non-plant objects.

What is
GPS?

Global
Positioning System, or GPS, is a method for accurately determining positions and
elevations almost anywhere on earth. It uses an array of satellites placed into
orbit by the U.S. government. In order to fix a position using GPS, a special
receiver and data processing circuitry are required. The receiver must make
contact with a minimum of 4 of the satellites. Accuracy can vary from
approximately ±30 feet or more for inexpensive hand held receivers, to somewhere on
the order of ±1 centimeter for sophisticated systems that use a base station
located at a known position on earth to provide a reference signal, a technique
known as "differential processing". The original user of GPS was the
U.S. military, for whom the system was initially created. But GPS technology is
now open and available to all, and it has found applications as diverse as tracking
the locations of transit vehicles to data collection for mapping.

GPS vs. GIS

These
acronyms are often confused. GPS is the Global Positioning System described
above. GIS stands for Geographic Information System, a generic term for a
sophisticated computer mapping system linked to a database - BG-Map, for
example.

Advantages of GPS vs.
Total Stations:

No
Need for Control Points:

To use
a total station you will need professionally surveyed ground markers, which are
used as fixed reference points (control points). You will need a control point
within line of sight to each plant to be mapped. And, each of these control
points must be within line of sight to at least one other control point. For
GPS mapping, control points are not required. This eliminates the initial cost
of hiring a surveyor to install the markers.

Reduced
Manpower:

To map
plants efficiently using a total station, a two-person team is normally
required. One person holds the optical target and reads plant labels while the
other person operates the total station. Mapping plants with a GPS is normally
a one-person operation.

Disadvantages of GPS vs.
Total Stations:

Adverse
Effects of Tree Canopies:

Tree
Canopies, especially if wet, can affect GPS measurements, reducing their
accuracy or increasing the time it takes to measure. The GPS receiver will warn
you if reliable data is not available at a certain position. You can move to
the side and take a measurement away from the tree, specifying an offset - a
distance and an angle - between the point you are at and the actual location of
the tree. You can measure this distance and angle using a tape measure and a
compass.Or, you can purchase a
laser rangefinder and a built-in electronic compass, which will automatically
provide the offset data to the GPS.

Periodic Loss of Satellite Signals:

At
any given position and time, signals from a sufficient number of satellites may
not be available.You can
determine ahead of time when satellite coverage will not be adequate.Your GPS can vendor can help you with
this.Keep in mind though that it
will sometimes be necessary to schedule your work around these periods of low
satellite availability.

The
Question of Accuracy

Although
GPS systems can provide high accuracy, it is not necessarily consistent.
Changes in signal conditions can cause results to vary.Also, the time it takes to get an
accurate plant position at a given point can increase greatly if signal
conditions are poor.In general,
you can expect GPS accuracy of less than one meter most of the time when using
real-time or post-processing correction and on the order of 1 centimeter most
of the time when using your own base station for correction.One-meter accuracy is adequate for
mapping most woody plants while it is insufficient for mapping smaller tightly
spaced plants.Despite this
limitation, a GPS may serve well by mapping the larger woody plants in a bed
whereas the smaller plants may be added manually by sketching in their positions
in relation to the woody plants and hardscape features.

Total
station accuracy does not vary as long as the equipment is properly
maintained.Accuracies on the
order of 1 - 10 centimeters can be consistently obtained with a total station,
depending on the skill of the operator.This is perfectly adequate for mapping both woody herbaceous plants.

Some
Factors to Consider Before Purchasing GPS

Site Conditions

Some sites as not well suited to
GPS, due to topography, tall buildings, large trees or sources of radio
interference.Before purchasing a
GPS, have the vendor demonstrate the equipment at various locations around your
site to confirm that it will perform as intended.

Availability of Correction Data

For best accuracy, GPS data must be processed using a source
of correction data available from an outside source or from your own base
station.Correction may be done
inside the GPS receiver itself (real-time correction) or later at the office
(post-processing). Sources of correction data include the following:

·Post-Processiong Using Community
Base stations -
located throughout the U.S. and some other countries - correction data can be
accessed via the Internet and used for correction of data by post-processing.Contact your GPS vendor for a list of
available base stations.

·Real-Time Correction -
Real-time correction data can be delivered to your GPS receiver via the internet.
You will need a mobile data plan to access this.

·Your
Own Base Station -
If other sources of correction data are not available or not practical, you
could set up your own base station at a known position on or near your grounds.
This means purchasing 2 GPS units, one to serve as a base station and one to be
used for mapping. Some GPS vendors may offer special pricing when you buy 2 GPS
units, so the cost is not doubled.Using your own base station for real-time correction can yield very high
accuracy results.

Send
Your Comments

Each
garden will have to determine which measurement technology best meets its
unique needs. For some, the solution will be a simple tape measure. Others will
opt for a total station or GPS. If you have field experience using GPS to map
plants, we'd like to hear from you. Please contact
BG-Map..